This disclosure relates to the fabrication of polymeric resin molds used to form molded plastic items. More specifically, it relates to the fabrication of polymeric resin molds made by a rapid prototyping process.
Molded plastic orthodontic alignment appliances (“aligners”) and their method of manufacture as mass-fabricated custom items are disclosed, for example, in U.S. Pat. No. 5,975,893 and US Patent Application Publication 2005/0082703, both of which are commonly assigned to the assignee of the subject invention, and the complete disclosures of which are both incorporated herein by reference. Plastic orthodontic appliances, of the type disclosed in the above-referenced documents, are made as a set of incremental position adjustment appliances that are used to realign or reposition a patient's teeth. The series of incremental position adjustment appliances is custom made for each patient during the fabrication process.
The appliances are typically made by thermal-forming a piece of plastic over a unique mold that represents an incremental position to which a patient's teeth are to be moved, which position corresponds to a particular incremental position adjustment appliance. The unique molds are manufactured by downloading digitized representations of three-dimensional CAD models of the molds to a rapid prototyping (“RP”) apparatus, such as, for example, stereolithography apparatus (“SLA”) or photolithography apparatus (“PLA”). Because each aligner is associated with a unique mold on which the aligner is fabricated, for the purposes of this disclosure, the molds themselves are considered to be mass-fabricated custom items.
The molding process requires that the molds have substantial structural rigidity and strength, thereby allowing them to withstand the pressures and stresses of the molding process without deformation. Thus, the molds have typically been made as solid forms, each of which is a three-dimensional model or replica of a patient's dental arch at a particular stage of treatment. The use of solid forms has two distinct disadvantages: (1) It requires the use of a considerable amount of polymeric resin material, which is expensive; and (2) because the RP machine builds the molds up layer by layer, the greater the volume of material used in each layer, the longer it takes to form each layer. To save on material costs, the completed molds may be hollowed out or “shelled” before they are cured, and the material removed for re-use. There is a limit, however, to the amount of material that can be removed without degrading the structural integrity of the molds, and the shelling process itself is costly and time-consuming. Thus, the conventional process for forming the RP molds is costly, both in the expense of the material and in production time and costs.
It would thus be advantageous to provide a method of making polymeric resin molds, such as SLA molds, that can create molds with sufficient strength and rigidity to be used in the subsequent molding process, and that can do so while reducing both the amount of polymeric resin material and the fabrication time needed to make the molds.